Silicon carbide (SiC) is high in heat resistance and mechanical strength and further exhibits excellent physical and chemical properties such as resistance to radiation and is therefore coming under attention as an environment-resistant semiconductor material. Further, in recent years, demand for SiC single crystal as a substrate material for a blue to ultraviolet short wavelength optical device, high frequency high voltage resistance electronic device, etc. has been rising. In applications of SiC single crystals to the semiconductor field, high quality single crystal having a large area is sought. In particular, in applications for substrates of high frequency devices etc., in addition to the quality of the crystal, possession of a high electrical resistance is sought.
In the past, on the laboratory scale, for example, SiC single crystal of a size enabling fabrication of a semiconductor chip has been obtained by the sublimation recrystallization method (Lely method). However, with this method, the obtained single crystal is small in area and not easy to control in its dimensions and shape and further in its crystal polytypes or impurity carrier concentration. On the other hand, another practice has been to use the chemical vapor deposition method (CVD method) for heteroepitaxial growth on a silicon (Si) or other different type of substrate so as to grow a cubic silicon carbide single crystal. With this method, a large area single crystal is obtained, but the lattice mismatch with the Si substrate is as high as about 20% etc., so it is only possible to grow an SiC single crystal including numerous defects (up to 107 cm−2) and it is not easy to obtain a high quality SiC single crystal. To solve these problems, the improved Lely method of using an SiC single crystal wafer as a seed crystal for sublimination recrystallization has been proposed (Yu. M. Tairov and V. F. Tsvetkov, J. Crystal Growth, vol. 52 (1981), pp. 146 to 150). If using this improved Lely method, it is possible to grow an SiC single crystal while controlling the crystal polytype (6H type, 4H type, 15R type, etc.) and the shape, carrier type, and concentration of the SiC single crystal. At the present time, SiC single crystal wafers of a size of 2 inch (50 mm) to 3 inch (75 mm) are being cut from SiC single crystal prepared by the improved Lely method and used for fabrication of devices in the electronics field etc.
On the other hand, in recent years, as a material for high frequency semiconductor devices, gallium nitride (GaN) having properties superior even to silicon (Si) or gallium arsenic (GaAs) is coming under attention (Rutberg & Co., Gallium Nitride: A Material Opportunity (2001)). In the fabrication of a GaN device, it is necessary to form a GaN single crystal thin film on some sort of single crystal substrate. As one general type of such a substrate, there is a sapphire substrate. Sapphire has the merit of enabling the stable supply of relatively good quality single crystal, but has a large difference in lattice constant from GaN of 13.8%, so easily induces a deterioration of the quality of the thin film formed on it. Further, the thermal conductivity is a small 0.42 W/cm·K, so there is also a problem in the point of dissipation of heat at the time of device operation. A GaN high frequency device formed on a sapphire substrate cannot currently be said to fully realize the inherent performance of GaN in quality and operating properties. As opposed to this, an SiC single crystal has a small difference of lattice constant with GaN of 3.4%, so a good quality GaN thin film can be formed. The thermal conductivity is also a large 3.3 W/cm·K, so the cooling efficiency is also high. Compared with sapphire and other conventional substrates, a great improvement in the properties of GaN devices can be expected. Therefore, in recent years, expectations have become very high for SiC single crystal substrates even in this field.
In high frequency device applications of the above-mentioned substrate, in addition to the quality of the crystal, reduction of the parasitic capacity of the chips fabricated on it and separation of the chips require that the substrate be raised in resistivity (to at least 5×103 Ωcm, preferably at least 1×105 Ωcm).
At the present time, such SiC high resistivity substrates are being industrially obtained by forming deep levels in the bandgap of the SiC single crystal by some sort of method. For example, it is known that vanadium forms deep levels in the SiC crystals in either the donor or acceptor state, compensates for a shallow donor or shallow acceptor impurity, and raises the crystal's resistivity. Specifically, for example, as shown in S. A. Reshanov et al., Materials Science Forum, vols. 353 to 356 (2001), pp. 53 to 56, in the above-mentioned sublimation recrystallization method, the SiC crystal powder material has added to it metal vanadium or a vanadium compound (silicate, oxide, etc.) which is made to sublimate together with the SiC material and thereby obtain a vanadium-doped crystal. However, the thus prepared SiC single crystal has a high resistivity, but is poor in crystal quality. Further, the crystal locations having a high resistivity constitute only extremely limited parts in the grown crystal.
Further, Japanese National Publication No. 9-500861 discloses the art of obtaining a higher resistivity vanadium-doped crystal. This art overcompensates for the impurity nitrogen in the SiC by addition of an element having a trivalent shallow acceptor level and changes the conductivity type from the n type to the p type to set the vanadium or other transition metal at the donor level and thereby obtain a high resistivity. However, even in the art of this publication, the problem of the vanadium concentration becoming uneven cannot be avoided. Even with this art, the inherent problems of crystal quality or yield in vanadium-doped crystals are not solved. Further, adding the acceptor element while controlling it to give the optimum concentration in the SiC crystal is difficult technically. Further, the concentration of the impurity nitrogen mixed into the SiC crystal in the sublimation recrystallization method generally changes during the growth by several orders of magnitude, so maintaining the optimum acceptor element concentration in the entire region of the SiC single crystal ingot can be said to be extremely difficult. For this reason, situations easily arise where a shortage of the acceptor element makes converting the crystal in conductivity type to the desired p type impossible or alternatively excessive addition of the acceptor element makes the crystal become an extreme p type and makes compensation by vanadium difficult. The art of this publication also does not solve the inherent problems of crystal quality and yield in vanadium-doped crystals.
The solid solution limit of vanadium to SiC is about 3 to 5×1017/cm3 or so. If the amount of vanadium exceeds the solid solution limit, as described in M. Bickermann et al., Materials Science Forum, vols. 389 to 393 (2002) pp. 139 to 142, there is the problem that a precipitate forms and the crystal quality drops. The amount of addition of vanadium is also restricted due to this reason, so in the prior art, high resistivity vanadium-doped crystal was difficult to produce.
On the other hand, it is also known that by reducing the concentration of the carrier impurity of the SiC single crystal down to a certain extremely low level, the crystal becomes high in resistivity. This is because the point defects of the deep levels present in the bandgap of the SiC crystals, called ID, UD-1, carbon vacancy, etc. trap the conductive electrons or holes (for example, M. E. Zvanut and V. V. Konovalov, Applied Physics Letters, Vol. 80, No. 3, pp. 410 to 412 (2002), B. Magnussen et al., Materials Science Forum, vols. 389 to 393 (2002) pp. 505 to 508). However, even the quality of the thus obtained high resistivity single crystal does not satisfy the high requirements of the semiconductor field at the present time.